The spread spectrum communication method is characterized in that it is strong against disturbance, noise and fading and has properties of concealing signals and secrecy and that asynchronous random access can be effected thereby, as described in "Spread Spectrum Systems" by R. C. Dixon. By this spread spectrum communication method, the initial synchronization (by which the synchronization between a PN code train contained in a received signal and a PN code train formed within a receiver is established with a high speed) and the synchronization holding (by which the synchronization is held stably) are necessary.
As an example of means for giving the initial synchronization described above, there is means disclosed in:
Literature 1: JP-B-Sho 64-11178 and on the other hand, as an example of means for giving the synchronization holding, there is known
Literature 2. Journal of Electronic Communication Society of Japan, 86/4 Vol. J69-B No. 4 pp. 403-405.
The means disclosed in Literature 1 concerning the initial synchronization is of type, by which a convolver is used as a correlation demodulator for the receiver. As it can be seen from a circuit of the receiver indicated in FIG. 5, in order to have the initial synchronization of the reference PN code train, time difference between the starting point of the reference PN code train and the starting point of the convolver output is measured; phase difference between the PN code train contained in the received signal and the reference PN code train is obtained from this time difference; and the phase of the reference PN code train is regulated so that this phase difference is removed.
The means disclosed in Literature 2 concerning to the synchronization molding is similarly of type, by which a convolver is used as a correlation demodulator for the receiver. As clearly seen from block diagrams of a transmitter and a receiver indicated in FIGS. 1A and 1B, respectively, it is characterized in that the phase difference between the PN code train contained in the received signal and the reference PN code train is converged to zero by initializing the phase of the reference PN code train in the correlation output.
Although the two methods for realizing the initial synchronization and the synchronization holding described above are useful in principle, in the case where they were used in a practical wireless propagation path, they had inconveniences that
a. the correlation output cannot be caught because of influences of noise and disturbance at effecting the initial synchronization; PA1 b. erroneous operations can be produced by a correlator output other than the aimed signal; etc. This is true also for the synchronization holding operation. PA1 a. the circuit is complicated, because the patter judging means, the digital filter, etc. are necessary; PA1 b. no satisfactory effect can be obtained, in the case where a threshold voltage for comparison used at forming two-valued data, because both the pattern judging means and the digital filter perform processing after the output of the correlator has been converted into two-valued data; PA1 c. since processing time by the pattern judging means should be increased in order to increase the effect of the means disclosed in Literature 3, as the result increase in the time required for the initial synchronization is produced; etc.
Therefore an example of improvement of the initializing operation is disclosed in;
Literature 3: JP-A-Hei 1-98338 and an example of improvement of the synchronization holding is disclosed in;
Literature 4: JP-A-Hei 1-98340.
In Literature 3 stated above, an initial synchronization pattern and a pattern for detecting information data start timing are included in transmitted data, and the output of the correlator is led to pattern judging means (i.e. matching filter) in order to assure further the initial synchronization operation, as shown by the circuit of the receiver indicated in FIG. 1B, so that influences of noise, etc. are reduced in this way. On the other hand, in Literature 4 concerning the synchronization holding stated above, it is intunded to reduce the influences of noise, etc. on the data demodulating operation by reducing the influences of noise on the synchronization operation by means of a digital filter (i.e. matching filter) and at the same time observing only the output of the correlator at the neighborhood of the point of time of the output of the aimed signal by using a window pulse.
However the devices disclosed in Literatures 3 and 4 had problematical points as follows;